1. Field of the Invention
The present invention relates to display drive devices and drive methods to drive displays such as liquid crystal displays. More specifically, the present invention relates to the improvement of drive output of display drive devices.
2. Description of Related Art
FIG. 21 schematically shows a structure of a flat display 700. The flat display 700 contains a flat display 710 such as a liquid crystal panel, an X-driver 720, a Y-driver 730 and a controller 740. The X-driver reads display data equivalent to one row of the display from data line 761 based on a predetermined control signal 760 from controller 740. Based on instructions from the controller 740, the Y-driver applies row selection voltage successively to scan electrodes for the 1st to Nth rows of the display 710. In association with this operation, the X-driver outputs voltage for display data of the one row to the signal electrodes for the 1st to Mth columns of the display 710. In this way, a display corresponding to display data is realized.
Now, let us consider conventional display drive devices (e.g., X-drivers). For example, let us consider Japanese Laid-open Patent Application HEI 4-107594. This reference shows the use of operational amplifiers, as indicated by reference numerals 10 and 13 in FIG. 1 and FIG. 3 of Patent Application HEI 4-107594. However, the amplifier only uses a transistor as a differential unit that has either an N-channel or a P-channel. Consequently, this operational amplifier presents a non-linear operation region 820 near the positive power supply potential, as shown in FIG. 22 of the present patent application, in the response of the output voltage to input voltage. With such a characteristic, even when a high voltage is to be output to a signal electrode in response to data with a high gray-scale level, a sufficient voltage cannot be applied to the signal electrode. As a result, it will be difficult to obtain a gray-scale display demanded. FIG. 22 of the present application shows an operational amplifier with an N-channel transistor for the differential unit, but using a P-channel transistor would also create a similar, albeit opposite, problem, as shown in FIG. 23 of the present application.
Japanese Laid-Open Patent Application HEI 8-122733 shows a different example of a drive method of a display drive device (an X-driver) in which voltages of the same polarity are applied to adjacent signal electrodes (e.g., 711 and 712 in FIG. 21 of the present application) and voltages of the opposite polarity are applied in the next display cycle. This reference describes an amplifier A (9) that absorbs current and an amplifier B (10) that releases current. A switch SA (11) and a switch SB (12) are opened or closed to selectively use the amplifier A (9) or the amplifier B (10) according to the voltage to be applied to CL. Since this structure entails discrete switching between the two types of amplifiers whose circuits are separate from one another, the following problem occurs. Namely, the open/close control between the switch SA (11) and the switch SB (12) would ideally be conducted such that one of them turns on while the other turns off, and such switching should take place simultaneously. However, strictly speaking, the possibility that there would be a moment when both of the switches are on or both of the switches are off cannot be denied. In such cases, there would be a problem of an output short-circuit or an output open-circuit. Further, even when the open/close control is to take place successfully, since the two amplifiers with individual differences are combined, a non-linear operation part 890 is created, as shown in FIG. 24 of the present application, resulting in display unevenness and display fluctuations.
In addition, although not shown in the figures, in a prior art circuit where an input voltage to an operational amplifier is selected, there is a possibility that a power supply voltage would be applied to individual transistors. Accordingly, transistors with a dielectric strength enough to withstand the power supply voltage have to be used. As a result, a large circuit area is required.
Furthermore, when using an operational amplifier with either a P-channel or an N-channel differential unit for one output, and forming a positive or a negative output by the differential output and a current source that operates by capacitive coupling with the differential output, the area required by a capacitor for the capacitive coupling has to be several times the area of the output transistor area, which consequently requires a large circuit area.
In a one-channel operational amplifier (900) in FIG. 25 of the present application, where an output is formed by a differential output (912) and a current source (930) operating with a capacitive coupling (920), the linear operation region of the output tends to become narrower. Consequently, either the power supply voltage Vdd has to be raised or the transistor""s threshold voltage has to be lowered. Additionally, because it requires a constant-current source (930) for the output, it was necessary to constantly supply a current to the output, which requires a large amount of current.
Therefore, the display device driving in the conventional technique has the following problems.
Namely, a proper voltage, that matches the gray-scale data, cannot be applied to liquid crystals in a non-linear operation region of the operational amplifier. As a result, a required gray-scale presentation cannot be obtained.
In a driving technique to cover a non-linear operation region by a pair of operational amplifiers having different polarities, problems, such as an output short circuit and an output open circuit, can occur.
In addition, when the operational amplifiers in the combination are switched from one to the other, a non-linear section appears in an output due to potential mismatching between the potentials at their junction, as shown in FIG. 24. As a result, a smooth gray-scale transition cannot be obtained, resulting in problems such as display non-uniformity and display fluctuation.
Also, transistors with a dielectric strength greater than the power supply voltage have to be used. As a result, a large circuit area is required. A circuit that requires a greater capacity also requires a greater circuit area. Also, in a circuit that requires a constant-current source (930), a large amount of current is consumed.
In accordance with one embodiment of the present invention, a display drive apparatus is provided for driving liquid crystals by applying a voltage to the liquid crystals based on gray-scale data representative of a display. The display drive apparatus includes a polarity control circuit that generates a polarity control signal based on a polarity signal; an input selection circuit that selects a predetermined reference voltage for driving the liquid crystals from a plurality of reference voltages provided from an input voltage generation circuit or a plurality of reference voltages generated within the input selection circuit based on an input signal representative of gray-scale data and the polarity control signal; a non-linear operation region identifying circuit that identifies a non-linear operation region in which an output signal from an operational amplifier does not follow an input thereto based on the input signal and the polarity control signal, and outputs an offset current generation control signal for supplementing the current of the output signal of the operational amplifier when an input voltage in the non-linear operation region is subjected to an operation amplification, wherein the operational amplifier inputs the preferred reference voltage selected by the input selection circuit and outputs at an output terminal an output signal that is operation-amplified according to the preferred reference voltage; and an offset current generation circuit that supplements the current at the output terminal of the operational amplifier based on the offset current generation control signal outputted from the nonlinear operation region identifying circuit.
As a result, a voltage in a non-linear operation region can be supplemented with a supplemental current that is provided by the offset current generation circuit, and a supplemented voltage is provided to an output terminal. Consequently, the linearity along the entire range between the low voltage side and the high voltage side can be secured, to thereby correctly apply a voltage that matches gray-scale data. Also, unlike the conventional technique, a large amount of current does not have to be constantly supplied. Furthermore, an operational amplifier provides an accurate output because output deviations due to performance deviations in transistors in the operational amplifier are mutually offset. In addition, the size of a differential unit can be reduced. Also, unlike the conventional technique in which the control of the operation amplifier is discrete, the present invention provides a continuous control over the operational amplifier, resulting in a higher operation speed.
In the display drive apparatus described above, the non-linear operation region identifying circuit outputs an offset current generation control signal in such a manner that the amount of the offset current is varied depending on the degree of deviation from a linearity of an input/output characteristic of the operational amplifier.
As a result, the operational amplifier provides an output closer to a linear characteristic in its non-linear region.
In the display drive apparatus described above, the non-linear operation region identifying circuit determines the offset current generation control signal based on the value of an input signal of gray-scale data.
As a result, the amount of the offset current can be corrected based on the data in advance, resulting in a linearity correction with a good response.
In the display drive apparatus described above, the non-linear operation region identifying circuit determines the offset current generation control signal based on predetermined upper bits of the input signal of gray-scale data.
Since the amount of offset current to be generated is determined by digital data, a relatively simple circuit structure can be implemented.
In the display drive apparatus described above, the non-linear operation region identifying circuit performs a conduction control for a transistor (423, 433) such that the offset voltage is selected when the polarity control signal has a predetermined value, and performs a conduction control for another transistor (426, 436) such that a power supply voltage having a polarity with which the offset current generation circuit does not conduct current is selected when the polarity control signal has a value having an opposite polarity with respect to that of the predetermined value.
As a result, problems such as output open circuit or output short circuit do not occur.
In the display drive apparatus described above, the polarity control circuit generates polarity control signals including a polarity control signal for even-numbered columns of the display and a polarity control signal for odd-numbered columns of the display.
The polarity control signals for the even-numbers columns and the odd-numbered columns can be provided to drive a liquid crystal display according to the dot-inversion control system.
In the display drive apparatus described above, the polarity control circuit generates a polarity control signal for even-numbered columns of the display and a polarity control signal for odd-numbered columns of the display that are mutually in logically inverted relation, depending on a logic level of the polarity signal, and further comprises a voltage changing circuit.
As a result, a polarity control signal can be transferred to a succeeding stage as a signal with a different electric signal level.
In the display drive apparatus described above, the operational amplifier has a first differential unit and a second differential unit having transistors with mutually opposite polarities. The offset current generation circuit and the second differential unit perform an amplification operation when an input to the operational amplifier is in a non-linear operation region of the first differential unit. The offset current generation circuit and the first differential unit perform an amplification operation when an input signal is in a non-linear operation region of the second differential unit. The first differential unit and the second differential unit perform an amplification operation in which the first differential unit and the second differential unit mutually supplement one another to provide an output when the first differential unit and the second differential unit are both in a linear operation region.
As a result, even when operational amplifiers have characteristic variations, they are mutually supplemented, and a linear characteristic can be obtained as a whole.
In the display drive apparatus described above, the offset current generation circuit is provided within the operational amplifier.
Since the offset current generation circuit is provided very close to an output terminal in which an offset current is supplied, the wire routing is simplified and the wiring impedance can be reduced.
In the display drive apparatus described above, the operational amplifier has a differential unit including a P-channel transistor and a differential unit having an N-channel transistor. The non-linear operation region identifying circuit and the offset current generation circuit provide an offset current to supply an electric charge to an output terminal of the operational amplifier when an output to be provided from the operational amplifier is at a low potential side, and provides an offset current to pull an electric charge from the output terminal when an output to be provided from the operational amplifier is at a high potential side.
As a result, an offset current does not always have to be supplied, and an intended object is achieved by supplying a minimum offset current.
The display drive apparatus described above is a semiconductor integrated circuit.
As a result, the display drive apparatus can be reduced in size, and can be readily mounted in a display apparatus.
The present invention includes a display apparatus equipped with the display drive apparatus and a liquid crystal display and a hand-carry type electronic apparatus equipped with the display apparatus.
The present invention can meet the requirements of display apparatuses and hand-carry type electronic apparatuses, such as small sized circuits, lower current consumption, less display unevenness and fewer display fluctuations.
Furthermore, in a display drive apparatus for driving liquid crystals by applying a voltage to the liquid crystals based on gray-scale data of a display, the apparatus includes a polarity control circuit that generates a polarity control signal based on a polarity signal; an input selection circuit that selects a predetermined reference voltage for driving the liquid crystals from a plurality of reference voltages provided from an input voltage generation circuit or a plurality of reference voltages generated within the input selection circuit based on an input signal representative of gray-scale data and the polarity control sinal; a non-linear operation region identifying circuit that inputs the predetermined reference voltage selected by the input selection circuit and identifies a non-linear operation region in which an output signal from an operational amplifier does not follow an input thereto based on the selected predetermined reference voltage, and outputs an offset current generation control signal for supplementing the current of the output signal of the operational amplifier when an input voltage in the non-linear operation region is subjected to an operation amplification, wherein the operational amplifier inputs the predetermined reference voltage selected by the input selection circuit and outputs at an output terminal an output signal that is operation-amplified according to the predetermined reference voltage; and an offset current generation circuit that supplements the current at the output terminal of the operational amplifier based on the offset current generation control signal outputted from the non-linear operation region identifying circuit.
As a result, the non-linear operation region identifying circuit directly inputs a voltage inputted in the operational amplifier and identifies the non-linear operation region. Accordingly, a more accurate linear correction can be achieved.
In addition, a display driving method for driving a display including selecting a reference voltage based on a gray-scale data, operation-amplifying the reference voltage to obtain a gray-scale for the display, changing voltages to be applied to liquid crystals based on the gray scale data to perform a driving operation, identifying a non-linear operation region in which an output signal from an operational amplifier does not follow an input thereto based on an input signal and a polarity control signal, outputting an offset current generation control signal for supplementing the current of the output signal of the operational amplifier when an input voltage in the non-linear operation region is subjected to an operation amplification, and supplementing an output signal from the operational amplifier with an offset current based on the offset current generation control signal when an input to the operational amplifier is at a low potential side or a high potential side.